AbstractBaseSession: Use model fields for subclassed cases (#2180)

* `AbstractBaseSession`: Use model fields for subclassed cases

In situations where these fields are overridden in custom models, for instance
extending 'session_key`'s `max_length`. Follow a similar style to `AuthBaseUser`.

See also:
- https://docs.djangoproject.com/en/5.0/topics/http/sessions/#extending-database-backed-session-engines
- https://github.com/django/django/blob/5.0.6/django/contrib/sessions/base_session.py
- https://github.com/typeddjango/django-stubs/blob/5.0.0/django-stubs/contrib/sessions/base_session.pyi#L13-L21

* `AbstractBaseSession`: Remove `objects` (declared in `Model`)

Co-authored-by: Marti Raudsepp <[email protected]>

* `AbstractBaseSession`: Use `ClassVar` (credit: @flaeppe)

---------

Co-authored-by: Marti Raudsepp <[email protected]>

django-stubs/contrib/sessions/base_session.pyi

@@ -1,8 +1,9 @@
 from datetime import datetime
-from typing import Any, TypeVar
+from typing import Any, ClassVar, TypeVar
 
 from django.contrib.sessions.backends.base import SessionBase
 from django.db import models
+from typing_extensions import Self
 
 _T = TypeVar("_T", bound=AbstractBaseSession)
 
@@ -11,10 +12,10 @@ class BaseSessionManager(models.Manager[_T]):
     def save(self, session_key: str, session_dict: dict[str, Any], expire_date: datetime) -> _T: ...
 
 class AbstractBaseSession(models.Model):
-    expire_date: datetime
-    session_data: str
-    session_key: str
-    objects: Any
+    session_key = models.CharField(primary_key=True)
+    session_data = models.TextField()
+    expire_date = models.DateTimeField()
+    objects: ClassVar[BaseSessionManager[Self]]
 
     @classmethod
     def get_session_store_class(cls) -> type[SessionBase] | None: ...

scripts/stubtest/allowlist_todo.txt

@@ -509,6 +509,7 @@ django.contrib.sessions.backends.signed_cookies.SessionStore.exists
 django.contrib.sessions.base_session.AbstractBaseSession.expire_date
 django.contrib.sessions.base_session.AbstractBaseSession.get_next_by_expire_date
 django.contrib.sessions.base_session.AbstractBaseSession.get_previous_by_expire_date
+django.contrib.sessions.base_session.AbstractBaseSession.objects
 django.contrib.sessions.base_session.AbstractBaseSession.session_data
 django.contrib.sessions.base_session.AbstractBaseSession.session_key
 django.contrib.sessions.base_session.BaseSessionManager.__slotnames__